Rebuilding generated protobuf code

If you make changes to .proto files, you will need to rebuild the generated Go code. To do that, you will need:

• The protobuf compiler
• A proper Go development setup per BUILD.md (variables GOPATH and GOBIN set properly)
• go get -u github.com/golang/protobuf/protoc-gen-go

Note: Newer releases of the protobuf compiler may be incompatible with the protobuf version in Beam. For guaranteed compatibility, use the latest release available from the date of the golang/protobuf release used by Beam. (Currently v3.5.2)

If all this setup is complete, simply run go generate in the current directory (pkg/beam/model).